Method and apparatus for treating material having poor thermal conductivity

ABSTRACT

A method and an apparatus for heating or cooling material having poor thermal conductivity, especially medium-consistency fiber suspensions. The material is directed, substantially as a plug flow, at a velocity below 5 m/s (e.g. 0.1-1 m/s) through an apparatus formed by a flow channel provided with heat exchange surfaces. The flow of the material is throttled at a throttling point by a more than 30% reduction in the cross-sectional area of the channel. After throttling, the material is discharged from the throttling point in such a manner that another portion of the material contacts the heat exchange surfaces.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. national phase of International applicationno. PCT/ FI99/00054 filed Jan. 28, 1999.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for treatingmaterial having poor thermal conductivity. The method and apparatusaccording to the invention are especially well applicable to heating orcooling of medium-consistency fiber suspensions within wood-processingindustry, or in more general terms to treatment of pulp. In particular,the method and apparatus according to the invention are applied toheating pulp having a consistency of 5-20%, preferably 6-16%, or torecovery of heat from the pulp. The method according to the invention issuitable for treating pulp for the bleaching process at a raisedtemperature, for example. Bleaching processes using high temperaturesinclude for instance oxygen and peroxide bleaching. Naturally, themethod and apparatus for the invention are also applicable to recoveringheat from the pulp or cooling the pulp.

It is known from the prior art that vapor is used for theabove-mentioned purposes, i.e. for heating the pulp for bleaching,whereby the pulp is heated directly with the vapor. A process like thisoperates in such a way that the pulp is supplied by means of a pump intoa vapor feeding device, in which it is possible by feeding vapordirectly into the pulp to raise the temperature of the pulp as desired.Subsequent to the mixing of vapor, the pulp is directed into a mixer, bymeans of which the temperature differences brought about in the mixingprocess are evened out and the desired bleaching chemical/s is/are mixedinto the pulp. From the mixer, the pulp is directed further into areactor tower, in which the bleaching process itself takes place. Inperoxide bleaching, for example, the temperature in the tower ismaintained at about 100° C. and the pressure in the lower part of thetower at about 10-8 bar and in the upper part of the tower at about 5-3bar. The pulp is removed from the tower by means of a removing deviceinto a blow tank, where the vapor still in the pulp is separated fromthe pulp to the upper part of the blow tank and from which the pulp isremoved by means of a pump. The vapor separated to the upper part of theblow tank is guided to a condenser, in which the heat still in the vaporis recovered from the vapor, the result being condensation water.

However, the process described above involves some disadvantages.

Firstly, a large part of the vapor is condensated into the pulp, wherebythe consistency of the pulp is no longer the same as it was when exitingfrom the pump. For example, raising the temperature by 20° C. withdirect vapor makes the consistency fall about 0.5%, which in some casescauses obvious problems in the process.

Secondly, the pressure in the vapor feeding device has to be limited toabout 9-10 bar, as (depending on the mill conditions) there might not bevapor at a higher pressure available, or at least not in such a way thatit could be easily directed to the bleaching plant.

Thirdly, a large combination of a blow tank, a pump and a condenser isrequired for recovering heat and guiding the pulp to the followingprocess stage.

Fourthly, the highest temperature of the condenser is 100° C., becausethe pressure is lowered to the outer air pressure.

Fifthly, the condensate water from the condenser is foul, because itcontains residues of bleaching chemicals and reaction products of thebleaching.

Sixthly, the high-pressure vapor means costs to the cellulose pulp mill.If there were less need for high-pressure vapor, a corresponding amountof energy could be sold to power plants, for example.

It was believed that all the above-mentioned problems would be solved ifit were possible to develop an indirect heat exchanger that would besuitable for use with consistent pulp. In other words, it would be adevice that would efficiently be able to both heat and cool consistentpulp having a tendency to flow as a uniform fiber net, i.e. as a socalled plug. These so called MC heat exchangers are described at leastin FI patent applications 781789, 943001, 945783, 953064, 954185, 955007as well as in international patent application PCT/FI96/00330 and FIpatents 67584 and 78131.

FI patent application 781789 discloses a large number of apparatusarrangements exploiting and applying fluidization of consistent pulp.This 1970's publication is based on the fluidization theory, which hasnot been further developed until recently. Over the past two decades, ithas been discovered that the theory forms a sound basis for furtherdevelopment, but at that time, i.e. at the end of the 1970's, it did notyet lead to any other practical applications than the so called MC pump.In other words, the various objects of use described were at a stage ofelementary ideas and have required a great deal of further study in thecase of each individual apparatus. Further investigations have,depending on the case, led to the development of the apparatus to acommercial product or the rejection of the idea as unfeasible. Theoperating idea of the indirect heat exchanger described in theabove-mentioned patent application is that the casing of a tubularapparatus is encircled by heat exchange channels, the casing of theapparatus forming the heat exchange surface. Inside the tube, at thelocation of the heat exchange surfaces, there is a rotor, by means ofwhich the fiber suspension flowing in the tube is fluidized. The idea isthat an intense turbulence is able to circulate each pulp particle sodose to the heat exchange surface that the temperature thereof would beable to change in a way depending on whether it is desirable to recoverheat from the pulp or to heat the pulp. It is not known to us whetherthis kind of apparatus has ever been experimented. In the light ofcontemporary knowledge, it is obvious that the apparatus does work ifthe flow rate in the tube is sufficiently slow. However, the idea hastwo weaknesses. Firstly, treating the pulp for a long time by means of afluidizator inevitably affects the paper technical properties of thepulp, such as the strength or average length of the fibers. Secondly,fluidization consumes such a great deal of energy that a heat exchangerbased on the operation of a mechanical fluidizator will never become aproduct that would be accepted by cellulose pulp mills.

The heat exchanger according to FI patent 78131 is relatively small insize and intended to be positioned for example before the bleachingtower or after it, either to heat pulp or to recover heat from it. Theessential thing in the apparatus described in the patent is that on theinlet side of the heat exchange elements, there is a fluidizing device,by means of which the pulp is made flow through the relatively narrowpasses of the compact heat exchanger. However, the fluidizator, which isa prerequisite for the operation of the exchanger, is in fact a problem,as it consumes a large amount of energy. Also, the structure is notapplicable to a large bleaching tower, the diameter of which would be inthe order of 5-10 meters, for example. It is not even imaginable that insuch a large tank, the pulp could be fluidized over the whole crosssection area thereof, as described in the FI patent. The energyconsumption would be enormous, and on the other hand, severalfluidizators would have to be used, whereby there would inevitably beproblems with structures. An apparent problem is also that since thepublication does not present any precise dimensioning instructions forthe heat exchanger, the pulp in the heat exchange channels forms a fibernet and the pulp will not be able to discharge from the apparatus, orthat it may not be possible to heat the pulp in the apparatus asdesired.

The greatest disadvantage of both above-mentioned apparatus is theenergy consumption due to the fluidizator that would have to becontinuously used in the apparatus. To eliminate the problem, theoperation of the apparatus should, at least primarily, be based on theplug flow of the pulp.

FI patent 67584 describes the above-mentioned arrangement applying saidplug flow, in which heat exchange surfaces are arranged in connectionwith the wall of the bleaching tower. In other words, the publicationdiscloses the idea that pulp could be heated or cooled in the bleachingtower. However, the application described in the publication isunfeasible, because it simply does not function. As the consistent pulprises or falls as a uniform column in a bleaching tower having adiameter of several meters, it would be impossible to heat the whole ofthe pulp when heating the surface layer. If the intention were to raisethe temperature of the pulp in the whole tower by merely raising thesurface temperature, the arrangement would only result in enormoustemperature differences.

FI patent application 943001 discloses various alternatives forarranging an indirect heat exchanger within the reactor tower. Unlike inthe above-mentioned FI patent 67584, the heat exchanger is formed byconcentrical annular heat exchange elements arranged inside the reactortower, into which heat exchange elements the heat exchange medium,preferably vapor, is directed. Each heat exchange element preferablycomprises two concentrical cylindrical casings connected to each otherby the ends thereof by means of end surfaces. Through a closed annularspace, the heat exchange medium flows from the inlet to the outlet,heating simultaneously the casing surfaces as well the pulp glidingalong the outer surface thereof. The heat exchange surfaces areconnected to each other preferably by the vicinity of the upper edgesthereof by means of preferably radial channels, through which the heatexchange medium is led into all annular elements. At the same time, saidchannels also act as bearers for the heat exchange elements. Preferably,on the opposite side of the tower, the lower edges of the heat exchangeelements are connected to each other by means of channels, through whichthe condensated vapor and the condensate water are led out of theelements and out of the tower.

As one embodiment, said FI patent application shows how the surface ofthe elements does not, by any means, have to be even but may be bent aswell. The intention is to improve the heating of the pulp in the annularflow channels between the elements by causing turbulence in the pulp,which turbulence mixes the pulp particles moving along the surfaces ofthe elements with the particles moving further in the channels.Furthermore, in one embodiment it is illustrated how the heat exchangeelements, the outermost of which is positioned in connection with thewall of the reactor tower, are provided, by the outer surface againstthe pulp, with either annular ribs parallel with the periphery, or withspiral ribs. The purpose of the ribs is to cause some turbulence in theflowing pulp in order that the pulp heated on the surface of theelements would mix with the pulp flowing further from the surface of theelements, whereby the pulp would be heated more evenly.

It has also been observed in the above-mentioned FI patent application943001 that by means of turbulence or the like brought about by the ribsarranged on the heat exchange surfaces it is not possible to conductheat very far from the heat exchange surfaces, but the distance inpractice will be 50-200 mm, depending on the intensity of the turbulenceand the velocity and consistency of the pulp. According to said patentapplication, the heat exchange surfaces, i.e. the elements, shouldconsequently be arranged at a distance of 200-250 mm from each other. Inpractice, this is often impossible, because the flow resistancegenerated by the heat exchange surfaces would be too intense. As anothersolution, several heat exchangers may be arranged in the reactor towerone after another in the direction of the flow. The heat exchangers maybe arranged for example in such a way that the diameters of the heatexchange elements of a first heat exchanger form a series of 650 mm,1,150 mm, 1,650 mm, 2,150 mm and so on. The diameter series of a secondheat exchanger is correspondingly 400 mm, 900 mm, 1,400 mm, 1,900 mm,2,400 mm and so on. In other words, from the first heat exchanger, pulprings are discharged that are 500 mm thick, except at the centerthereof. Each of these rings is divided into two parts by means ofsecond heat exchange elements in such a way that the distance of the newdivision surface from the heated pulp layer, or rather from the surfaceagainst the second heat exchange elements, is 250 mm. In other words,the pulp is divided into slices, each of which is heated in turn.

After more thorough investigations into the matter, it was observed thatnot even the indirect heat exchanger within the tower, which wasdisclosed in FI patent application 943001, was reliable. It has beennoticed, for example, that if many separate annular heat exchangeelements are disposed within the tower, there is a great risk that thepulp flow will channel at some points of the tower between the heatexchange elements in such a way that most of the heat exchange surfacescannot be utilized. In other words, at least in the light ofcontemporary studies it seems that the heating of pulp by means ofseveral heat exchange rings positioned within each other would not bepossible, but the heating ought to be carried out in a separateapparatus of a smaller size.

Furthermore, the experiments carried out show that the pulp layer of 250mm presented in said publication is far too thick to be heatedindirectly. Thereafter, a solution has been sought with reference totreatment of much thinner pulp layers.

In the prior art publication referred to in the following, i.e. in theinternational patent application PCT/FI 96/00330, the invention is basedon determining some mediumconsistency pulp properties not preciselyknown beforehand with such accuracy that it has become possible tooptimize the operation of the apparatus utilizing these properties, sothat the apparatus have become industrially useable. Whereas in ourearlier patent application 943001 it was believed that heating could becarried out in a pulp layer of about 250 mm, the performed study showedthat heat is generated, practically speaking, only at a distance ofabout 10-30 mm from the surface of the heat exchanger. Further, it wasobserved in the study that the flow rate of the pulp has to be in therange of 0.01-5 m/s, preferably 0.1-1 m/s, and most preferably 0.1-0.5m/s. The next observation was that the length of the heat exchangesurface in the flow direction of the pulp should be in the order of10-70 cm in order to heat said pulp layer as effectively as possible.Therefore, a heat exchanger according to the invention comprises asubstantially cylindrical flow channel, i.e. a tube, in which there maybe a heat exchange channevs arranged at least on part of the peripherythereof, preferably encircling the whole tube. A number of heat exchangeelements located preferably on the diameter of the tube are arranged oneafter another inside the tube. The elements are disposed in the tube insuch a way that they divide the pulp plug flowing in the tube into twoparts, so that at the length of the whole set of elements the pulp plugbecomes divided into equal sectors, for example into 60-degree sectors,forming a star-like figure seen from the direction of the shaft.Preferably, the elements are located closely one after another, so thatthere will be no substantial changes in the flow cross section whenmoving from the area of one element to that of another. The heatexchange elements preferably comprises two opposite plates, and there isa channel for heat exchange medium therebetween.

Experiments indicated that the heat exchanger operated as expected.However, the most difficult practical problem turned out to be thecomplex structure of the heat exchanger, which makes the apparatusunreasonably expensive.

To eliminate the above-menboned problem, among other things, developmentwork was started to design a heat exchanger with a simpler structure. Atthe same time, the intention was to try out an operating principle thatwas somewhat different from that of the conventional indirect heatexchangers. Experiments on previous versions of heat exchangers hadyielded so much new information about the behavior of medium-consistencypulp in a complete plug flow and in the vicinity thereof that it was nowtime to try out the heat exchanger in the area of partial plug flow.

Another problem observed in the experiments was that the outer surfaceof the tube always heats the same pulp. Therefore, a simple method tochange the pulp flowing along the tube wall was needed. This issurprisingly easy to carry out by arranging throttling points to theflow. After the throttling point the pulp flows out again onto the innersurface of the tube, but this time it is other pulp particles that arelikely to encounter the inner surface of the pipe than those flowingalong it before the throttling point. A throttling point required in amethod and apparatus according to our invention closes more than 30%,preferably more than 50% of the flow channel, and most preferably morethan 70% of the flow channel. Hereby, the flow rate of the pulp in thethrottling point is 1.5-2-fold, preferably over threefold compared to anormal tube flow.

The throttling point is preferably slot-like, but also many other formsare applicable, such as a circle, a half-circle, an ellipse, a rectangleand a triangle. The essential thing is that the throttling point changesthe pulp flow in such a way that a new layer of pulp encounters thesurface of the tube.

The flow rate of the pulp in the flow channel between the throttlingpoints is 0.01-5 m/s, preferably 0.1-1.0 m/s, and more preferably0.1-0.5 m/s. At the throttling points the flow rate is over 1.5-fold,preferably over threefold. more than 30%, preferably more than 50% ofthe flow channel, and most preferably more than 70% of the flow channel.Hereby, the flow rate of the pulp in the throttling point is 1.5-2-fold,preferably over threefold compared to a normal tube flow.

The throttling point is preferably slot-like, but also many other formsare applicable, such as a circle, a half-circle, an ellipse, a rectangleand a triangle. The essential thing is that the throttling point changesthe pulp flow in such a way that a new layer of pulp encounters thesurface of the tube.

The flow rate of the pulp in the flow channel between the throttlingpoints is 0.01-5 m/s, preferably 0.1-1.0 m/s, and more preferably0.1-0.5 m/s. At the throttling points the flow rate is over 1.5-fold,preferably over threefold.

At the throttling point the pulp is partly mixed, but it is stillpreferable that after the last throttling point there is a mixer thatevens out the temperature differences in the pulp. The mixer may beself-rotating in the flow or provided with a separate operating device.Of course, the mixer may also be used for mixing chemicals into thepulp. The length of the heat exchange surface between the throttlingpoints is greater than 10 cm, usually 10-200 cm, preferably about 10-70cm.

GB-A-2 135 439 discusses a heat exchanger for lubricating oils. The heatexchanger is formed of a lengthy pipe being divided into sections byinternal baffle elements. The baffle elements are shaped so that theyare able to exchange the laminar boundary layer flowing along thepipewall with the stream flowing in the middle of the pipe. Theoperation of the elements is based on allowing a thin boundary layerflowing as a first flow along the pipewall to flow beneath a firstbaffle element whereas the rest of the flow so called second flow isguided by means of said first baffle element to the center of the pipe.The first flow is, then, directed sharply towards the center of the pipeby means of a second baffle element arranged perpendicular to thepipewall. The purpose of the second baffle element is to force the firstflow through the second flow to the center of the pipe whereas thesecond flow from the center would, then, form a new boundary layer.

Characterizng features of an apparatus eliminating the above-mentionedproblems of the prior art and attaining the above-mentioned purposes ofthe invention become apparent from the appended claims.

It is characterizing to one preferred embodiment of the method andapparatus for heating and cooling pulp by means of indirect heatexchange surfaces according to the invention that the pulp is allowed toflow in a closed flow channel at a consistency of 5-20%, preferably8-15%. Hereby, the flow channel comprises at least two throttlingpoints, in which the flow rate of the pulp rises at least 50%,preferably 100%, and even more preferably 150%. Between the throttlingpoints and before the first throttling point there is a heat exchangesurface in the surface of the flow channel, the length of which heatexchange surface is more than 10 cm but less than 500 cm, preferablyless than 100 cm, In addition to the throttling points there may beother changes made to the pipe to change the geometry of the pipe,usually for increasing the heating surface area. Thus there may be,before the throttling point, a pipe extension or a change from roundpipe to, for example, rectangular pipe. Also inside walls or dividingwalls etc. may be inserted before or after a throttling point.

The present invention is a result of a long-term series of experimentsstudying the behavior of the medium-consistent pulp; the experimentshave deepened the understanding in the field to such an extent that ithas become possible to develop apparatus that no one would have believedcould operate only a few years ago. An example of the studies is a heatexchanger, in which medium-consistency pulp can be heated or cooledcompletely without a fluidizing apparatus, if desired. What makes theinvention especially significant is that the apparatus is applicable toalmost countless objects of use in a cellulose pulp mill.

Some of the advantages of the method and apparatus according to theinvention were theoretically achievable already with the apparatus ofthe above-mentioned FI patent application 943001, but in this context itis especially worth mentioning that

the consistency of the pulp does not change when heating the pulp,

the condensation water remains pure and can be recycled,

neither the pressure in the reactor nor the temperature of the condenserneeds to be limited according to the requirements of the vapor,

there is no need for a large blow tower-pump-condensator combination,

the pressure of the pulp in the reactor tower may be used to feed thepulp to the following process stage, for example into a washer,

low-pressure vapor may be used for heating the pulp; such vapor isnormally classified as waste in cellulose pulp mills, so that itsremoval and condensation have to be arranged in any case. By utilizingthe amount of heat present in the low-pressure vapor by means of anindirect heat exchanger according to our invention it becomes possibleto sell a larger part of the energy produced by the mill,

the apparatus has a spacious and simple structure,

the large inner surface of the tube functions as a heat exchangeelement, and

there being only one flow channel, the pulp flow in the apparatus doesnot channel but proceeds uniformly through the apparatus.

One of the advantages that could also be mentioned is that the innersurface of the flow channel acts as a primary heat exchange surface, theinner surface being always relatively large. When the channel iscircular, the following areas are achieved, presuming that the distancebetween the throttlings is 0.5 meter. With one throttling, the heatexchange surface area of the tube preceding the throttling point is¶*D*L=¶*1*0.5=1.5 m² and the heat exchange surface area following thethrottling point is the same, i.e. 3 m² altogether. Correspondingly,with two throttling points, there is 3+1.5=4.5 of heat exchange surfaceand with three throttling points 4.5+1.5=6 m². Thus, with fivethrottlings, a heat exchange surface area of 9 m² is achieved. Areassuch as these are sufficient for raising the temperature of the pulp bymore than 5° C., preferably by more than 10° C. It is typical of themethod of the apparatus that the change in the temperature is less than50° C., preferably less than 20° C., sometimes even less than 10° C.

Further, it is characterizing to the apparatus according to oneembodiment of the invention that the diameter of the flow channel ismore than 0.5 m, preferably more than 1.0 m, but less than 3 m, andpreferably less than 1.5 m.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the method and apparatus according to the inventionare described in more detail with reference to the attached figures, ofwhich

FIG. 1 illustrates an apparatus according to one preferred embodiment ofthe invention as an axial section;

FIG. 2 illustrates an apparatus according to FIG. 1 as a section A—A;

FIG. 3 illustrates an apparatus according to another preferredembodiment of the invention as an axial section; and

FIG. 4 illustrates an apparatus according to FIG. 3 as a section B—B.

DETAILED DESCRIPTION OF THE DRAWINGS

An apparatus 10 shown in FIGS. 1 and 2 according to a preferredembodiment of the invention for treating material that has poor thermalconductivity, i.e. for heating or cooling the material, comprises a tube12 preferably having a circular diameter, which tube is provided by theends thereof with flanges 14 to attach the apparatus 10 to a tube lineor the like. Inside said tube 12 there are two heat exchange elements 16and 18 arranged on the opposite sides of the tube, which heat exchangeelements throttle the cross sectional area of the tubeone-dimensionally. Said heat exchange elements 16 and 18 are preferablyidentical, being preferably formed of plate material bent in a desiredform. In the embodiment of FIGS. 1 and 2, said heat exchange elements 16and 18 are cut into such a form that the surfaces 161, 162, 181 and 182thereof remain as planes when the heat exchange elements have beenattached inside the tube 12. Between the heat exchange element 16 andthe tube 12 there is a vapor space 163. Likewise, there is a vapor space183 between the heat exchange element 18 and the tube 12. The heatexchange elements 16 and 18 are dimensioned in such a way that thereremains an opening of an even width between the elements in the middlepart of the tube, the cross section of which is substantiallyrectangular, the cross sectional area being about 30-70% of the wholecross sectional area of the tube.

In the embodiment of FIGS. 1 and 2, two pairs of heat exchange elements16, 18 are arranged inside the tube one after another in such a way thatthe openings between the pairs are perpendicular relative to each other.Outside the tube 12, at a distance from the tube 12, there ispreferably, although not necessarily, a heat-insulated casing 20,arranged in such a way that there is a vapor space between the tube 12and the casing 20. Hereby, the whole area of the tube may be used forheating pulp and for recovering heat from pulp. The vapor is led intothe inside spaces 163 and 183 of the heat exchange elements 16 and 18preferably from the vapor space encircling the tube. Correspondingly,the recovery of the condensate may be arranged either together from thecondensate removal from the vapor space of the tube, or if desired,along separate conduits.

The apparatus according to FIGS. 1 and 2 operates in such a way thatmedium-consistency fiber suspension to be treated is supplied into theapparatus 10 from the left (FIG. 1). The flow rate of the pulp in theapparatus is below 5 m/s, preferably below 1 m/s, most preferably0.1-1.0 m/s. As the pulp proceeds as a plug inside the tube 12, the plugbumps against the surfaces 162 and 182. Due to the pressure of the pulpcoming into the apparatus, the plug flow breaks up at the location ofthe surfaces, whereby the pulp discharges through the opening betweenthe surfaces in a turbulent state. Hereby, having glided along thesurfaces 162 and 182 and having been heated on the surfaces, the pulpbreaks up into particles, which are mixed with the pulp flow dischargingthrough the opening between the heat exchange elements 16 and 18.Corresponding mixing takes place also in the opposite direction. Inother words, the pulp having flown in the middle part of the tube 12breaks up into particles in the opening between the heat exchangeelements 16 and 18 and mixes into the pulp so that part of saidparticles drift against the surfaces 161 and 181, whereby also theseparticles will be heated. When the pulp proceeds in the tube 12 and theflow cross sectional area increases at the location of the surfaces 161and 181, the pulp forms a new plug flow, whereby the above-describedoperation is repeated at the location of the following pair of heatexchange elements 16 and 18. Now however, as the heat exchange elements16 and 18 are disposed, as seen from the end of the tube 12 (FIG. 2), ina perpendicular position relative to the preceding pair of heat exchangeelements, it is ensured that the pulp flowing in the tube becomes mixedalong the length of the apparatus. Hereby, the major part of the flowwill at some phase be in contact with the heat exchange surfaces.

FIGS. 3 and 4 illustrate an apparatus according to another preferredembodiment of the invention. The main structure of the apparatus is asin the embodiment of the FIGS. 1 and 2. The only significant differenceis that the surfaces 16 and 18 of the heat exchange elements are curvedone-dimensionally. In other words, the end view of the apparatusillustrated in FIG. 4 is similar to that in the embodiment of FIG. 2,i.e. the opening between the heat exchange elements is substantiallyrectangular, the plane forming the surface of the heat exchange elements16 and 18 has been bent one-dimensionally only. The heat exchangeelements 16 and 18 comprise in this embodiment, as seen from theincoming direction of the flow, concave surfaces 164 and 184, convexsurfaces 165 and 185, between which a flow opening is formed, andconcave surfaces 163 and 183. Bending the surfaces has mostly to do withthe strength of materials; bent surfaces have a better tolerance of thestress the apparatus is subjected to, i.e. pressure and temperaturevariations.

In addition to the above-mentioned structural arrangements, which arethe most preferable from the point of view of manufacturing techniqueand in which the pairs of heat exchange elements comprise a plate thatonly requires bending and cutting into an appropriate form, there arenaturally other structural solutions, in which a three-dimensionalobject is formed of the plate material. In fact, FIG. 3 is a relativelygood illustration of the form of heat exchange elements also in the caseof a three-dimensional plate. In other words, an object resembling ahalf-circle to some extent is pressed from the plate (corresponding tothe plates 164 and 184 of the heat exchange elements), in the middle ofwhich an opening of a desired size is opened. In the same way, athree-dimensional plate corresponding to the plates 163 and 183 of theheat exchange elements is produced, and an opening of a desired size islikewise opened in the middle of the plate. The objects produced in thisway are attached to each other either directly by the edge of theopening in the middle, or by means of a connecting means. Naturally, theform of the opening in the middle may be different from the presumedannular opening; it can be an ellipse or even a polygon, for example.

Above, a tube is described as a means having a heatable casing, insideof which two pairs of heat exchange elements are arranged one afteranother and at an angle of 90 degrees relative to each other, but otherkinds of structures are also possible. At its simplest form, theapparatus is formed by a cylinder tube provided with end flanges, insideof which cylinder tube there is one pair of heat exchangers. Byattaching a sufficient number of these kinds of devices one afteranother and taking into account the transition, i.e. the varying angularsetting to be arranged between the heat exchange members in apparatusarranged one after another, it is possible to heat pulp at a desiredtemperature. Naturally, the next complex solution would involve addingheat insulation upon the cylinder tube, and in the next version it wouldbe possible to arrange a possibility for heating, i.e. a vapor casing,between the tube and the heat insulator. Further, it is possible toconstruct an apparatus with three pairs of heat exchange elements. Insuch a case, it is preferable to arrange the angular difference betweenthe heat exchange elements to be 60 degrees.

In the apparatus according to our invention, the throttling point usedin the apparatus is slot-like, but many other forms are also applicable,such as a circle, a half-circle, an ellipse, a rectangle or a triangle.The essential thing is that the throttling point changes the pulp flowin such a way that a new layer of pulp encounters the surface of thetube. In the experiments, it has been observed that a suitable flow ratein the flow channel between the throttling points is 0.01-5 m/s,preferably 0.1-1.0 m/s, and more preferably 0.1-0.5 m/s. In thethrottling point, the flow rate is 1.5-fold, preferably over 3-fold.

Although the pulp is partly mixed at the throttling points, it is stillpreferable that after the last throttling point there is a mixer eveningout the temperature differences in the pulp. In the apparatus accordingto our invention, the mixer may be either self-rotating in the flow orprovided with a separate operating device. Of course, the mixer may alsobe used for mixing chemicals into the pulp.

The experiments have shown that the distance between the throttlings ofthe heat exchange surface is preferably less than 500 cm, preferablyless than 100 cm, and more preferably about 10-70 cm. Correspondingly,an appropriate diameter for the flow channel in an apparatus accordingto a preferred embodiment of the invention is more than 0.5 m,preferably more than 1.0 m, but less than 3 m, preferably less than 1.5m. With this dimensioning, the channel being circular, the followingheat exchange surface areas are achieved at a one-meter tube diameter,presuming that the distance between the throttlings is 0.5 m. With onethrottling, the heat exchange surface area of the tube preceding thethrottling point is ¶*D*L=¶*1*0.5=1.5 m², and the heat exchange surfacearea following the throttling point is the same, i.e. 3 m² altogether.Correspondingly, with two throttling points there is 3+1.5=4.5 m² of theheat exchange surface area, and with three throttlings 4.5+1.5=6 m².Thus, for example, with five throttlings a heat exchange surface area of9 m² is achieved. These areas are sufficient to change the temperatureof the pulp by over 5° C., even over 10° C. It is typical of the methodaccording to the invention that the change in the temperature is below50° C., preferably below 20° C., sometimes even less than 100° C.

According to yet another embodiment of the invention the diameter of theflow pipe may, however, be as small as 20 cm in cases where the flowchannel has been positioned between two reaction towers or liketreatment vessels. Normally, in such cases where the only purpose of theflow channel Is to deliver the pulp to another treatment vessel thediameter varies between 20 and 60 cm.

As can be seen from the above description, it has been possible todevelop such an indirect heat exchanger for heating and cooling of pulpthat has a very simple structure and is therefore very reliable andpreferable. Only a few preferred embodiments of the invention aredescribed above, and it is to be taken into account that many apparatusdetails may in the final commercial product be significantly differentfrom the above structural arrangements, which are more of a schematicnature.

What is claimed is:
 1. A method of heating or cooling a pulp and paperindustry fiber suspension having a consistency of 5-20% using aplurality of heat exchange surfaces defining a single flow channelhaving an inner diameter of between 0.2-3 m, comprising: a) directingthe suspension as a uniform, substantially plug, flow at a flow ratebelow 5 m/s into the single flow channel so that a first portion of thesuspension contacts and gives up heat to, or takes heat from, the heatexchange surfaces; b) at a first throttling location, throttling theflow of the suspension by causing a more than 30% reduction in the flowchannel cross-sectional area, where the cross-sectional area iscontiguous; c) after b) widening the single flow channel so that thesuspension is again directed as a uniform, substantially plug, flow at aflow rate below 5 m/s into the single flow channel so that a secondportion of the suspension, different than the first portion, contactsand gives up heat to, or takes heat from, the heat exchange surfaces;and d) repeating b) at least once at least one subsequent throttlinglocation downstream of the first location, including a second throttlinglocation spaced about 0.1-2 m from the first location.
 2. A method asrecited in claim 1 wherein d) is repeated at other throttling locationsdownstream of the second throttling location, the subsequent throttlinglocations spaced about 0.1-2 m from the second location or from eachother.
 3. A method as recited in claim 1 wherein a)-d) are practiced tochange the temperature of the fiber suspension between 5-20 degrees C.4. A method as recited in claim 1 wherein b) is practiced to reduce thedimension of the single flow channel in a first direction, whilemaintaining the dimension of the flow channel constant in a seconddirection perpendicular to the first direction.
 5. A method as recitedin claim 1 wherein b) and d) are practiced to increase the single flowat each of the throttling points by at least 50%.
 6. A method as recitedin claim 1 wherein a) and c) are practiced so that the uniform,substantially plug, flow of the fiber suspension is at a flow ratebetween 0.1-1.0 m/s.
 7. A method as recited in claim 1 wherein b) and d)are practiced by causing a more than 50% reduction in the flow channelcross-sectional area.
 8. A method as recited in claim 1 wherein b) andd) are practiced by causing a more than 70% reduction in the flowchannel cross-sectional area.
 9. A method as recited in claim 1 whereina) and c) are practiced utilizing a flow channel that has a diameterbetween 1-1.5 m.
 10. A method as recited in claim 1 wherein d) ispracticed so that the throttling locations are spaced between 0.1-0.7 m.11. Apparatus for heating or cooling a fiber suspension comprising: asingle flow channel having a contiguous cross-sectional area, and aninterior diameter of between 0.2-3 m, said single flow channel providedwith a plurality of interior heat exchange surfaces, and having a firstend and a second end; and at least one throttling location disposedalong said single flow channel between said first and second endsthereof, said throttling location causing a more than 30% reduction inthe cross-sectional area of said flow channel.
 12. Apparatus as recitedin claim 11 wherein said at least one throttling location comprises aplurality of throttling locations, said throttling locations spaced fromeach other along said single flow channel a distance of between 0.1-2 m.13. Apparatus as recited in claim 11 wherein said at least onethrottling location is provided at least in part by said heat exchangesurfaces.
 14. Apparatus as recited in claim 11 wherein said single flowchannel at said at least one throttling location defines a contiguousflow opening that is substantially rectangular, semi-circular,elliptical, or triangular in cross-section.
 15. Apparatus as recited inclaim 11 wherein said throttling location is defined by two oppositesubstantially parallel restricting elements.
 16. Apparatus as recited inclaim 15 wherein said substantially parallel elements comprise platesattached to opposite walls of said flow channel.
 17. Apparatus asrecited in claim 11 wherein at said throttling location a plate having acurved surface is provided attached onto a wall of said single flowchannel.
 18. Apparatus as recited in claim 11 wherein at said at leastone throttling location there is a more than 70% reduction in thecross-sectional area of said single flow channel.
 19. Apparatus asrecited in claim 12 wherein at each of said throttling locations thereis a more than 50% reduction in the contiguous cross-sectional area ofsaid flow channel, and wherein said throttling locations are spaced fromeach other along said flow channel a distance of between 0.1-0.7 m. 20.Apparatus as recited in claim 11 wherein said single flow channelcomprises a tube having an exterior wall, and a vapor space. 21.Apparatus as recited in claim 19 wherein said flow channel has aninterior diameter of between 1-1.5 m.
 22. A method of heating or coolinga pulp and paper industry fiber suspension having a consistency of 8-15%using a plurality of heat exchange surfaces defining a single flowchannel having an inner diameter of between 0.2-3 m, comprising: a)directing the suspension as a uniform, substantially plug, flow at aflow rate between 0.1-1.0 m/s into the single flow channel so that afirst portion of the suspension contacts and gives up heat to, or takesheat from, the heat exchange surfaces; b) at least one throttlinglocation, throttling the flow of the suspension by causing a more than50% reduction in the flow channel contiguous cross-sectional area; c)after b) widening the flow channel so that the suspension is againdirected as a uniform, substantially plug, flow at a flow rate below 5m/s into the single flow channel so that a second portion of thesuspension, different than the first portion, contacts and gives up heatto, or takes heat from, the heat exchange surfaces; and wherein a)-c)are practiced so as to change the temperature of the fiber suspensionbetween 5-20 degrees C.